It is known that modern internal combustion engines are provided with a fuel injection system for directly injecting the fuel into the cylinders of the engine. As an example, the so called Common Rail System (CRS) is ubiquitous for Diesel Engines. The CRS, generally, includes a fuel pump, hydraulically connected to a fuel common rail and a plurality of electrically controlled fuel injectors, which are individually located in a respective cylinder of the engine and which are hydraulically connected to the fuel rail through dedicated injection pipes.
As also known, the injection pressure, which may reach values of about 200 MPa, is an important parameter for determining the quality of the fuel injection into the engine (for example, the fuel spray penetration in the cylinder head). Hence, any measurement or estimation of the injection pressure has to be accurate as much as possible. Currently, the injection pressure is measured by means of an analog pressure sensor, which is located on the common rail. Therefore, the terms “injection pressure” and “common rail pressure” are used synonymously in the present disclosure.
Recently, engine manufacturers are deciding to use a digital pressure sensor to measure the injection pressure, instead of the analog sensor. This solution will be compliant with the OBD2 requirements, common for both gasoline and diesel engines and will save one pin of the Electronic Control Unit (ECU).
The pressure measurements are managed by the ECU, using a time-based task (for example, 6.25 ms) for the pressure control and an angular-based task based on the engine crankshaft, triggered close to the first incoming injection, for the fuel injection control, in order to improve the fuel injected quantity accuracy. The angular-based task position is scheduled by the time-based task on an engine angular base. For engine angular base is meant that a time interval is replaced by the angle, the engine crankshaft has realized in the same time interval. Using the digital pressure sensor, the injection pressure sampling, which is time-based and is used for the injection control and, particularly, for the injector energizing time (ET) calculation function, happens inside a period in an unpredictable position with respect to the angular-based task. Being that the rail pressure behavior strongly affected by oscillations of its values, the unpredictable position penalizes the injection accuracy, increasing the injector quantity deviation.
Therefore a need exists for a method of estimating the injection pressure for an internal combustion engine, without the above inaccuracy, wherein the pressure signal is provided by a digital pressure sensor.